1. Technical Field of the Invention
The present invention relates generally to rotating electrical machines, such as electric generators and motors. More particularly, the invention relates to a rotor for an automotive alternator which includes a mechanism for positioning magnetic pole cores, and a method of manufacturing the rotor.
2. Description of the Related Art
There is known, for example from U.S. Pat. No. 6,486,585 B1, an automotive alternator that includes a plurality of permanent magnets each of which is interposed between two adjacent magnetic pole claws of a rotor to reduce the magnetic flux leakage between the two magnetic pole claws. With the reduction in the magnetic flux leakage, an increased amount of magnetic flux will be accordingly introduced from the rotor to a stator, thereby increasing the output power of the alternator.
However, referring to FIG. 14, in the automotive alternator disclosed in U.S. Pat. No. 6,486,585 B1, the magnetic pole claws are formed by: first assembling a pair of magnetic pole cores 2 and 3 onto a rotary shaft (not shown) of the rotor; and then machining the magnetic pole cores 2 and 3, leaving collar portions 2a and 3a on the respective outer peripheries of the magnetic pole cores 2 and 3. In addition, in FIG. 14, the reference numeral 24 indicates a milling tool that is used for machining the magnetic pole cores 2 and 3.
Accordingly, if the magnetic pole cores 2 and 3 could not be accurately positioned in the circumferential direction of the rotary shaft in the assembly step, the intervals between adjacent pairs of the magnetic pole claws in the circumferential direction would vary greatly. Thus, for a particular adjacent pair of the magnetic pole claws which have a large interval therebetween, the respective collar portions 2a and 2b would be left small in the subsequent machining step. Consequently, during high-speed rotation of the rotor, it would be difficult to securely hold the permanent magnet (not shown) interposed between the pair of the magnetic pole claws against centrifugal force. As a result, the permanent magnet would fly out of the space between the pair of the magnetic pole claws, causing a failure of the automotive alternator; for example, the permanent magnet may damage other components of the alternator, such as the stator and the field coil, or be caught in a small gap between the rotor and the stator, thereby locking the rotor.
To solve the above problem, one may consider machining the magnetic pole cores 2 and 3 prior to assembling those onto the rotary shaft. However, in this case, it is still necessary to accurately position the magnetic pole cores 2 and 3 in the circumferential direction in the assembly step, so as to make the intervals between adjacent pairs of the magnetic pole claws even. Accordingly, depending on the way of positioning the magnetic pole cores 2 and 3, a guidance device may be required to guide the magnetic pole cores 2 and 3 to desired positions in the assembly step, making the manufacturing process complicated and thus increasing the manufacturing cost of the rotor.